Prof. Maskelyne and Dr. Lang's Mineralogical Notes. 433 



For the twin crystals* we have the angles 



101(100)101 = 57° 6 

 104(100)104=11 46 

 103(100)103=16 38 

 102(100)102 = 26 12 



The combinations observed belong to two types. The crystals 

 of the first type, as represented in figs. 1 to 4, are elongated 

 parallel to the axis of the prism (1 10); the planes at their lower 

 end, by which they rest on the mass, could not be observed, and 

 the crystals appear therefore in the drawings terminated only by 

 the very perfect cleavage-plane 101. The crystals of this group 

 are all twinned, the twin face being 100: the two individuals 

 penetrate one another sometimes in a very complex manner, 

 which may often cause the unevenness of the prism-planes. I 

 succeeded several times in splitting out of such a twin crystal a 

 form of an apparently prismatic character, such as is represented 

 in a section parallel to the plane of symmetry in fig. 5, where the 

 perfect cleavage-planes are marked with the letter c. But a slide 

 parallel to this section showed in polarized light the true struc- 

 ture, as the parts marked with the same number become dark at 

 the same azimuth between two crossed Nicol prisms. Similar 

 penetrations of two twinned crystals are found also on other 

 oblique minerals, as for instance in gypsum. Such were also 

 lately described by G. vom Rath on crystals of epidote. The 

 forms in figs. 1, 3, 4 were observed on specimens occurring in a 

 kind of sandstone from Wallaroo and Burra Burra in South 

 Australia. These specimens are either very small crystals of 

 malachite of a light green colour, disseminated on chrysocolla, 

 or the crystals on them are larger and of a darker colour, and 

 occur in the hollows of denser accumulations of crystallized ma- 

 lachite together with blue carbonate of copper. Fig. 2 gives the 

 form of small deep green crystals of malachite from Grimberg 

 near Siegen, on limonite. 



Fig. 1.— 110, 112, 10 2, 101. The following angles 

 were observed : — 



01 



. 1 2 = 42 



(41 39calc.) 



12 



.102 = 12 



(LI 39 „ ) 



10 



1 1 2 = 73 



(72 34 „ ) 



12 



.ll0 = 86| 



(87 28 „ ) 



* It seems to me more convenient to give in this form the angles be- 

 tween planes of two twin individuals, putting the plane (or zone-axis) on 

 which they are twinned in the middle, than, as has been usual hitherto, to 

 write the plane of the second individual with inverted letters. 



